A database typically refers to a central storage location for information that a user of a program, such as a database management system, can access, share, and/or otherwise modify. Despite best maintenance practices, databases can still fail. Common examples of failure can include system crashes, user error, statement failure, application software errors, network failure, and media failure. Hardware malfunction, a bug in the database software, or the operating system itself are common instances that could cause one of the failures described above. Each time a database fails, a user of a database can lose information and/or halt transactions being conducted on the failed database.
High Availability and Disaster Recovery (HADR) refers to data replication technology that provides a high availability solution for both partial and complete failures of a database. HADR protects against data loss on databases by performing a process known as failover. The failover process replicates data changes from a source database, called the primary, to a target database, called the standby. In the case of failure, the failover of databases hosted on the primary server to the standby server typically occurs in sequential order. On average, each database failover process takes several minutes. Thus, users of a database can experience a down time of a few minutes or longer before databases are failed over and input-output can be resumed.